The present invention concerns a heat exchanger made at least in part of aluminum or its alloys and a method of its fabrication. More specifically, this invention provides a method suitable to use for an evaporator of a cooling/heating system such as evaporator for car air conditioners.
Generally, the heat exchange area of a heat exchanger is designed with as large a surface area as possible, while for the purpose of reducing the size, it is designed with large volume air flow and extremely narrow interfin spacing. For this reason, the atmospheric humidity condenses to form water at the fin interspaces. Water condensation takes place at the cold side of the heat exchanger during either heating or cooling.
Water condensed due to the above-mentioned cause and accumulated in the interfin spaces increases the ventilation resistance, which results in performance degradation. It also leads to rusting of fins giving rise to a problem of pollution due to the disengagement of corrosion products from the surface.
Water condensing in interfin spaces and in the condensation tray tends to be blown off by the heat exchanger air blower and as a result corrodes the periphery of the heat exchanger.
Consequently it is desirable to reduce the amount of water droplets in the interfin spaces so as to avoid fin clogging due to water. For this purpose it is desired to provide the aluminum fin surface with a good hydrophilic property in order to improve its water wettability.
In regard to the method of forming hydrophilic surface on the fins, application of a coating which comprises fine silica particles, silicate, calcium carbonate and aluminum hydroxide sol has been publicly known. However, the adhesion of such inorganic substances in the coating tends to deteriorate as time in service elapses disengaging finely pulverized particles which stimulate the human olfactory organ to yield an unpleasant smell. Further these coatings have a tendency to lower the hydrophilic performance. There is also a method including a resin in the coating of silica etc., in order to prevent the disengagement of silica particles. In many cases, however, the hydrophilic performance tends to lower as resin proportion increases. This comes from the general properties that an organic high molecular resin has, i.e. it is hard for a resin coating to have a contact angle with water lower than 30.degree., which leads to the difficulty of getting a good water-wettable surface. For this reason, increasing the resin proportion to the extent at which disengagement of inorganic substances may be prevented, makes it hard to attain the desired hydrophilic property. It is also publicly known to make supplementary addition of surfactant to the resin, but surfactant dissolves in water, tends to be washed away and is unfavorable for maintaining the hydrophilic surface at a constant level.
In addition, the surface of aluminum provided with hydrophilic property through the above-mentioned treatments is, in general, apt to corrode in water, and tends to be washed away. Consequently as service time elapses, in many cases the hydrophilic quality is impaired.
Heretofore, chromate conversion treatment has been known as a treatment of aluminum-made heat exchangers for the purpose of corrosion prevention. Chromate conversion coating has water wettability for the initial time period of coating formation. But, since its property changes from hydrophilic to hydrophobic with time, chromate conversion coating cannot be used as a method for hydrophilic surface formation; its function has merely been to provide corrosion resistance. As methods for surface treatment capable of providing corrosion resistance besides chromate conversion treatment, there are anodic oxidation treatment, resin coating treatment etc. which are known to us. However, the coatings obtained with these methods also are hydrophobic.
To mention the method of manufacturing aluminum heat exchangers, there are two methods which have been used. One is cutting to size of aluminum coil, punching and forming, welding for assemblying parts to heat exchanger body which then is subjected to the above-mentioned corrosion resistance surface treatment or to a corrosion resistance surface treatment as a base coat over which then is formed a hydrophilic coating. The other is to use aluminum coil previously surface treated, i.e., so-called precoated aluminum coil, which is cut to size, then subjected to punching/forming. However, the hydrophilic coating formed by these conventional surface treatments is insufficient in durability and gives rise to problems in degradation of the heat exchanging performance and corrosion development. This particularly applies to exchangers to be used for both cooling and heating cycle such as those for a car air conditioner where there is also required reduction of size as well as large air volume. Further in connection with the heat exchanger for a car air conditioner, in many cases assemblying fins and tubes to the unit is done by brazing the joint portions and is followed by chemical conversion treatment and hydrophilic surface treatment. In this case, Al-Si brazing filler metal of high Si content is applied to the fin core metal which in advance is clad with brazing filler metal, after which excessive brazing filler metal is removed by hot water spray; however, there remains on the surface of the aluminum base metal a eutectic of aluminum and silicon, on which chemical conversion coating is difficult to deposit. This results in the corrosion resistance of the fin being insufficient.
The object of the present invention is to provide a heat exchanger made of aluminum which does not give rise to the problem that fine particles scatter around from the surface treatment coating as well as to offer a method for the fabrication of such heat exchanger through which the corrosion resistance and hydrophilic property of aluminum base metal thereof are improved.